Humidity... High to low?

ChrisJ

I have a question regarding infiltration.

Humidity..... It goes from high to low and it's very hard to stop.

Ok but what does high to low mean?  Is it based on relative humidity or dew point?

If two spaces have the same dew point but one has a much lower RH will moisture migrate or are they balanced?

neilc

wetter RH will try and get to your lower RH
High RH to Low

ChrisJ

neilc said:

wetter RH will try and get to your lower RH High RH to Low

So if you have a conditioned space that's 72f and 50% rh the humidity from that will try to migrate into the attic which may be 130f and 15% rh?

neilc

this is turning into a phsyco metric chart brain teaser,
and I'm not sure without the chart, or a calculator and the algebra,

Air / vapor sealing between the conditioned space and the attic , to stop the humidity passing is what you're after,

I'm thinking the low RH# in the attic is because the heat is that high,
~50 inside is fair to good,
what the RH outdoors? that's gotta be close to the heated RH in the attic

Jamie Hall

It isn't the relative humidity which migrates -- unless you are transferring capillary water in a wick or wick like setting of some kind -- it's the air carrying the water vapour. And the air can go either way, depending on the relative pressures (the pressure differences can be very small).

ChrisJ

Jamie Hall said:

It isn't the relative humidity which migrates -- unless you are transferring capillary water in a wick or wick like setting of some kind -- it's the air carrying the water vapour. And the air can go either way, depending on the relative pressures (the pressure differences can be very small).

So humidity doesn't migrate on it's own?  It must be moved via air?

Jamie Hall

ChrisJ said:

Jamie Hall said:

It isn't the relative humidity which migrates -- unless you are transferring capillary water in a wick or wick like setting of some kind -- it's the air carrying the water vapour. And the air can go either way, depending on the relative pressures (the pressure differences can be very small).

So humidity doesn't migrate on it's own?  It must be moved via air?

Just so. "Humidity" is actually a term for the mass fraction of water vapour in the air, and relative humidity is simply that actual mass fraction divided by the maximum mass fraction that can be there without condensing. Hot air can hold more water vapour without condensing than cooler air can -- so if a parcel of air warms up, the relative humidity goes down and vice versa. The actual mass of water vapour per unit volume doesn't change.

ChrisJ

Jamie Hall said:

Jamie Hall said:

It isn't the relative humidity which migrates -- unless you are transferring capillary water in a wick or wick like setting of some kind -- it's the air carrying the water vapour. And the air can go either way, depending on the relative pressures (the pressure differences can be very small).

So humidity doesn't migrate on it's own?  It must be moved via air?

Just so. "Humidity" is actually a term for the mass fraction of water vapour in the air, and relative humidity is simply that actual mass fraction divided by the maximum mass fraction that can be there without condensing. Hot air can hold more water vapour without condensing than cooler air can -- so if a parcel of air warms up, the relative humidity goes down and vice versa. The actual mass of water vapour per unit volume doesn't change.

So on an antique refrigerator getting moisture in the insulation was a big deal.  They initially tried sealing everything tight but found this was impossible and water still found its way.  So they started sealing them to the outside world and leaving the gaps and even vents later on open to the inside.  This allowed moisture to migrate out of the cabinet walls and insulation and onto the evaporator where it would be removed when defrosted.

How is that moisture ending up in the cabinet walls if they were reasonably sealed so no air movement.  And how is it getting back out to the evaporator?

Obviously this is a bit different than a house or structure as your dealing with no less than a 40 degree difference and as much as a 100 degree difference.

But none the less...... apparently it happens because I originally had water problems with the monitor top and they vanished when I got rid of the interior gasket.

By sealed I'm talking closed cell foam gaskets, wax soaked cloth gaskets and wax on bolt threads.   Far far better sealing than could ever be done in a house.

ch4man

ChrisJ said:

neilc said:

wetter RH will try and get to your lower RH
High RH to Low

So if you have a conditioned space that's 72f and 50% rh the humidity from that will try to migrate into the attic which may be 130f and 15% rh?

72'/50%=58.61 grains per lb. but 130'/15%= 101.78 grains per lb.
so..your attic will will lose that fight as neither is fully saturated
I think......

neilc

so the attic is wetter, and hotter,
and will do its best to get into the house